Technical SBC 8.2 mpg ?

I’m swapping carbs today on the 55. Prior to pulling it apart, I topped off the tank and crunched the numbers, suspecting my mileage was pretty awful. It was not awful, it was holy crap awful. 8.2 mpg.
So the new motor (350cui, 300 hp, GM crate engine) , with less than 70 miles on it, is until today, running an Edlebrock 750, (transferred from the old engine). I’m installing a new 650 cfm now.
Odd thing is it seemed to run pretty good, but the gas gauge was in a death dive. So other than being over carbureted, I also see my in-line fuel pressure gauge is bouncing floating around 8-10 psi at idle. Edlebrock recommends 6psi max and suggests 5.5. I’m surprised my stock mechanical pump is pushing that much pressure. Could the excess pressure contribute to the suck gas mileage? Maybe blow past the float needle?
I was in Autozone earlier and they had a pressure regulator in stock, but it said adjustable between 1–4 psi. Hmmm. So too low with a regulator, and too high without it.
New carb should be on later today, so curious to see if the mileage is still atrocious. I don’t think the excess pressure will hurt the carb…

 

Trans and rear ratio?

 

What’s the oil smell like? Are the plugs black? I’d guess if the fuel was dribbling in it and through you’d not have much or any air bleed adjustment?

 

About 5 lbs for Edelbrock carb. Have found that the 290hP crate is not good on mileage-may be due to low compression-not sure. Put smaller chamber heads on one with a Crane 266/440 cam and it woke it up a good bit--don't know if mileage improved though.

 

Yes, too much fuel pressure.

The pump off the old engine?

 

That is a strange post to see - I was asking myself that yesterday! International Harevster truck -327 SBC ,350 auto trans,3.38 ish rear with tall tyres .I think I am similar MPG!!! I have a Weber carb 500cfm with a regulator running at 4psi . I think the main jetting may be to large-smells a bit rich so a jet kit and some meddling may be in order

 

>>>I topped off the tank and crunched the numbers>>>

Gas pump was probably set to liters. So you actually got 31 miles per gallon. Pretty good.

 

That's not bad compared to some 440s I've had in the past

 

tell us about the gauge. Does it read Zero when there's no pressure?

Tell us about the pump. Does it have 2 or 3 fittings on it? if 3, what's connected to the return (small) fitting?

 

Muncie with a 3.27 rear gear

 

Yep. I do believe I was noticing bad mileage on the old engine before it ate itself.

 

Standard GM mechanical pump. Inlet from the tank and a single output.
Running, the gauge pulses along with the diaphragm usually between 8-10 psi. When I shut the engine off, it holds randomly between 8-10 lbs, depending I guess where the pump diaphragm ended up. If I bleed off pressure by loosening a line, it drops to zero.

 

Interesting.

This type regulator

https://www.holley.com/products/fue...regulators/carbureted_regulators/parts/12-803

is probably what you'd need to control the pressure, and have adequate flow. The little ones with a dial don't flow much fuel.

If it's any consolation, I get less than 8 in my little Chevy driving around town. It will get 10 on the road.

 

“Not good” and God awful are two differ thangs..

 

I would expect 8 to 10 around town with "spirited" driving. 12-15 on the road. So it's not really out of whack.

Also, going by one part tank fillup is not real accurate...averaging over several full tanks will give you a more accurate number.

 

Define
a. Little Chevy
b. Driving around town…lol
I’ve been known to drive a “little” Chevy around town with a heavy foot.

 

I only traveled 64 miles and burned 7.8 gallons. Just initial test drive and then to the alignment shop. I’m dreaming of 16mpg….

 

But it might burn your car down. I know someone who's over-pressurized carb overflowed out a vent and started a fire. Luckily only the carb needed replacing and the hood repainted.

Then he's installing a regulator and gauge.

 

Awesome. Looks exactly like the one at Autozone, but with a pressure range where I need it.

 

Regulator Squirrel shows is what we use. The little ones with the dial-no thx.

 

It's a damn hot rod ,did you build it for economy ????? Trade it in for a Honda
LOL

 

One of the perks of living in Canada , driving season is 5 months , the other 7 we save for fuel !!!!

 

I have tuned a couple of these and they all need a lot of timing initial and vacuum advance

 

Time it with a vacuum gauge.

 

Thinking of a nice Prius...LOL.

 

No kidding, 327, 30 over, camel hump heads, flat top pistons, 273 gears, turbo 350, 20-25 mpg consistently, always has, and yes, it'll get out of its own way. Sees 90% hiway stuff.

 

I lot of folks do not realize there is a low pressure version of that regulator and a standard one.

My similar 350 with a cheap/new 650 Holley got 18 to 20ish all the time. Rear was 3.23 with a T400

 

That's what I use.
Off the shelf replacement mechanical pumps can put out way too much pressure, so never assume just because it's a mechanical pump the pressure will be okay. You can waste a lot of time and money chasing engine issues and blaming carburetors if the pressure is to high in the first place.

 

Check around the intake/carb base for a vacuum leak......(voice of experience with a Y-block engine.)

 

Air/fuel ratio much too rich, I presume. Do you read the plugs? I have never had to use a regulator on a carburetor in almost 50 years with mechanical pump. The regulator issue came up when Edelbrock came out with their 'tuning' perspectives. Holley's have never let me down when floats were set correctly at idle. Use a vacuum gauge and set timing and idle mixture for maximum vacuum. Carb size is not the issue, it's the jetting and most likely ignition timing.

Joe